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Changing the lights on communication towers may be all it takes to significantly reduce the number of bird deaths that result from night-time collisions, says a US study.

All we have to do, suggests the study, is change the bulbs from steady to blinking lights.

"The potential is that all the communication towers in the world could potentially be changed to be better," says Terry Rich, director of the US-based Partners in Flight, a bird conservation group, who was not involved with the study.

An estimated 4 or 5 million birds die each year in the United States from flying into communications towers, says study lead author Joelle Gehring, though some estimates range as high as 50 million birds a year.

Seeing stars

Migratory songbirds are the most common victims of tower collisions, with most run-ins occurring at night. Scientists suspect that migratory birds use stellar constellations and other light cues to navigate.

If the animals encounter other lights along their journey, they can become easily confused, especially if fog, clouds or rain push them down to lower altitudes and obstruct their view of the stars.

Confused birds often circle illuminated towers before ramming into them or the wires that connect them to the ground.

The structures are lit at night so that pilots can avoid them.

"We knew these towers were involved in collisions for decades," says Gehring, a conservation scientist at the Michigan Natural Features Inventory in Lansing. "But we never dove into why and how we could prevent it."

Gehring and colleagues happened upon the perfect natural experiment. The Michigan state police maintain a network of communication towers that were built around the same time and are all equipped with the same lighting systems.

The police department agreed to give the researchers full access to the towers, including the ability to adjust the way the lights glowed at night.

The scientists randomly selected 21 towers around the state. All of the towers were about 140 metres tall.

Significant difference

Each was assigned to one of four lighting scenarios: Blinking white lights; blinking red lights; red lights that alternate between bright and dim without ever shutting off completely; and steady-burning red lights that don't blink at all.

For 20 days, a network of researchers and volunteers walked around each tower at the same time every morning, picking up bird carcasses and sending them to Gehring for counting and analysis.

The final tally revealed that the type of lighting used had a huge impact on bird deaths.

"Those towers that had steady-burning red lights had significantly more collisions than all of the other three combined," says Gehring. "We could reduce avian collisions by as much as 70%, just by turning off those steady-burning lights."

For now, the US Federal Aviation Administration approves only steady-burning reds and blinking whites. But those policies are outdated, says Gehring, and the FAA is planning to reassess the safety of other scenarios.

It would be not only easy and cheap to switch steady reds over to blinking reds, she says, the change would also save energy and reduce maintenance costs.